Syngas, a mixture of carbon monoxide and hydrogen, is widely employed in the catalytic production of hydrocarbons, alcohols and mixtures of these products.
Typically for the production of saturated hydrocarbons and olefins the reaction is carried out in a reaction inert liquid medium containing suspended catalyst. Typical catalysts include iron and cobalt normally absorbed on a carrier such as thorium, magnesium oxide or kieselguhr. To produce alcohols, noble metals on the same supports are often used.
The usual source of syngas is the gasification reaction in which water vapor is reacted with a source of carbon, usually coal, under known conditions. The reaction is endothermic and a common source of heat for the reaction is to burn some of the coal by reaction with oxygen. The theoretical reaction including the burning is: EQU 2C+H.sub.2 O+1/2O.sub.2 .music-sharp.2CO+H.sub.2
so that the theoretical ratio of carbon monoxide to hydrogen in the reaction product is 2:1.
For the production of methane, the theoretical reaction is: EQU CO+3H.sub.2 .music-sharp.H--CH.sub.2 --H+H.sub.2 O
and the general reaction for the production of higher saturated hydrocarbons is: EQU nCO+(2n+1)H.sub.2 .music-sharp.H--CH.sub.2 --.sub.n H+nH.sub.2 O
It is thus apparent that the ideal ratio of hydrogen to carbon monoxide in feed gas for the production of saturated hydrocarbons is between 2:1 and 3:1. The theoretical ratio in syngas, as shown above is only 1:2. In fact the "real world" actual ratio in the usual industrial operation is about 0.5:1 to 0.8:1 in syngas.
To compensate for the missing hydrogen, the usual commercial practice is to mix the syngas with water vapor as an additional hydrogen source.
The theoretical reaction which takes place in this case is represented by the equation: ##STR1## where n is the number of carbon atoms in the saturated hydrocarbon produced, and x is the number of moles of water.
To conduct this reaction at maximum efficiency using the low ratio syngas industrially available, iron and cobalt are the most widely employed catalysts. Iron is advantageous because it is "flexible" which means that it will accept and operate reasonably efficiently with low ratio syngas in which the ratio of hydrogen to carbon monoxide varies over a wide range. However, iron requires the use of high temperatures and the rate of reaction is relatively low. The use of cobalt gives rise to higher reaction rates, but cobalt is not flexible and is sensitive to temperatures above 250.degree. C.
The generalized equation for the production of olefinic hydrocarbons is: ##STR2##
The problems experienced with this reaction are similar to the problems experienced for the production of saturated hydrocarbons.
When a syngas-water mixture is employed to produce alkanols, the theoretical equation is: ##STR3##
The catalysts normally employed in the reaction have been palladium, platinum, iridium, copper and ruthenium with or without a carrier. These catalysts have proved to be flexible like iron, but they also suffer the same disadvantages as iron. They require high temperatures, and the rate of reaction is low.